1. Field of the Invention
The invention relates to an apparatus for the production of plastic compound pipes.
2. Background Art
An injection head for an apparatus for the production of plastic compound pipes with exterior transverse grooves is known from DE 29 11 833 C3, in which an intermediate pipe is provided in an injection channel, having an upstream cutting edge for dividing a flow of melt and succeeding guide surfaces. In this way, the flow of melt is divided into flows enclosing each other concentrically. For identical speed of discharge of these two flows of melt from a downstream internal nozzle or from a downstream external nozzle to be reached, heating and/or cooling devices are provided. This is meant to render possible in particular the working of PVC into compound pipes which have a smooth interior pipe and corrugated exterior pipe. In this known apparatus, it cannot be avoided that the interior pipe and/or the corrugated exterior pipe of the compound pipe have a varying wall thickness over the circumference.
An apparatus for the production of compound pipes of the generic type is known from EP 208 055 B2, in which the injection head comprises an internal channel and an external channel. The width of the external channel can be modified externally over the circumference by means of a set collar. The width of the internal channel can be modified externally by the internal mandrel being adjusted by means of screws. As a prerequisite of the adjustability of the internal mandrel, the entire system must be strongly braced axially for leakage to be avoided. As a result, high adjusting forces are necessary. If leakage occurs in spite of this, then the melt flowing out of the internal channel streams inwards where it is burnt. This gives rise to considerable cleaning problems.
An apparatus for the production of compound pipes is known from U.S. Pat. No. 5,346,384, the injection head of which comprises a nozzle body to which, among other things, an external nozzle jacket is attached by means of an adjusting device. By means of this adjusting device, this external nozzle jacket can be adjusted radially to the centrally longitudinal axis so that the width of the external channel defined by this external nozzle jacket can be adjusted over the circumference of the external channel in such a way that a melt hose emerges from a downstream external nozzle that has a constant thickness over its circumference. In particular in the production of compound pipes of greater diameters, corresponding adjustment of the flow of melt that produces the interior pipe of the compound pipe is not possible.
The prospectus "Rollepaal" of the company of Machinefactory `De Rollepaal` bv of 7701 Dedemsvaart, the Netherlands, teaches to provide ultrasonic wall-thickness-measuring devices for the production of compound pipes. When the pipe wall thickness deviates from a given desired value, centering of the hose forming the pipe takes place in the nozzle by means of heatings, the effect typical of PVC being utilized, namely that the sliding behavior of the PVC melt on a wall, and thus the throughput of the melt through the nozzle, strongly depends on the wall temperature.